The present invention relates to a variable-capacitance radial turbine having a swingable tongue member for varying a sectional area of an upstream throat portion of a turbine scroll fluid passage.
In general, an operating characteristic of an internal combustion engine equipped with a turbocharger is approximately determined by a ratio of an area A of the section of the turbine scroll throat portion, to a distance R between the turbine wheel axis and the center of gravity of the section of the scroll throat portion. A turbine having the ratio A/R of a high value is suitable for increasing the engine torque at high engine speeds.
A variable-capacitance radial turbine of a type to which the present invention relates, has a swingable tongue member which is arranged to change the sectional area A of the turbine scroll throat portion, and thereby provide an adequate supercharging not only at high engine speeds but also at low engine speeds. One example of such a variable-capacitance radial turbine is disclosed in Japanese Patent provisional publication No. 54-84123.
However, the swingable tongue member and the turbine scroll throat portion receiving the tongue member are liable to disturb the exhaust gas flow in the turbine scroll passage and reduce the turbine efficiency. Especially, the clearances which are formed between the tongue member and the scroll side walls in order to allow thermal expansion of the tongue member having a thermal expansion coefficient different from that of the turbine housing, are undesirable to the turbine efficiency. When the tongue member is in an outer limit position to minimize the sectional area A of the turbine scroll throat portion, the exhaust gases pass through these clearances from the upstream side to the downstream side in such a manner as to bypass the scroll passage, and flow into the normal exhaust gas flow coming through the scroll passage, so that the throttling effect of the tongue member is reduced, and the fluid energy is lost uselessly. When the tongue member is in an inner limit position to maximize the sectional area of the scroll throat portion, the exhaust gases pass through these clearances radially outwardly from the downstream side into the upstream throat portion, and this exhaust gas flow reduces the effect of the wide open tongue member and disturbs the fluid flow in the throat portion.